Old introduction

If you are about to begin developing PHP or Zend extensions, you need
to prepare yourself for the programming environment provided by the
various APIs. This part of the documentation tries to introduce the APIs
provided by the different PHP and Zend Engine versions available.
Since most of the information available here is somewhat outdated,
you'll want to read various files found in the PHP source, files such
as README.SELF-CONTAINED-EXTENSIONS and
README.EXT_SKEL in addition to the manual.